The invention relates to a device for transmitting television pictures and to a device for receiving television pictures thus transmitted. Both devices may be combined in one apparatus, constituting a video recorder. The invention also relates to a storage medium on which television pictures are stored.
A device for transmitting television pictures of the type described in the opening paragraph is known from European Patent Application EP 0 419 752. The known device comprises means for dividing each television picture into blocks, means for forming a motion vector for selected blocks, means for forming a reference vector and a plurality of difference vector from the motion vectors of a plurality of contiguous blocks which constitute a sub-picture, and a variable-length encoder for encoding the difference vectors into code words of variable length.
In the known device the reference vector is constituted by the mean value of the motion vectors of the sub-picture. The reference vector is transmitted as a code word of fixed length. The difference vectors are transmitted as code words of variable length. With this, a certain extent of encoding efficiency and protection from channel errors is aimed at.
It is an object of the invention to further improve the encoding efficiency of the device.
According to the invention the device is therefore characterized in that the motion vectors of a sub-picture are applied to the variable-length encoder in the form of a series which successively comprises the difference vectors and the reference vector, the variable-length encoder being of a type which allocates a code word of variable length to a series of zero values and a subsequent non-zero value.
The invention is based on the recognition that moving objects in a picture are generally larger than one block. This means that the blocks of a sub-picture generally comprise the same quantity of motion. The difference vectors then have a value of zero, whereas the reference vector is often unequal to zero. By presenting such a series of zero values and a subsequent non-zero value to the variable-length encoder, it will often compress the motion information of a sub-picture to one code word only.
The reference vector is preferably constituted by the motion vector of a selected block of the sub-picture. It is then no longer necessary to transmit a difference vector for this block so that the encoding efficiency further increases. The difference vectors are preferably formed from the motion vectors of contiguous blocks. The difference in motion of a block with a contiguous block is smaller than with a more remote reference block because contiguous blocks generally have a larger extent of corresponding motion. The difference vectors thus more frequently have the value of zero and are then encoded more efficiently.
In a preferred embodiment of the device the variable-length encoder is constituted by the variable-length encoder for encoding coefficients which are obtained from picture transform of a block. A separate variable-length encoder can thus be dispensed with. Moreover, it has been found that the series of applied motion vectors statistically corresponds satisfactorily to the distribution of coefficients within a block in which series of zero coefficients are often followed by a non-zero coefficient. The encoding efficiency is thus even further improved.
A corresponding device for receiving television pictures is characterized in that it comprises a variable-length decoder of a type which decodes a code word of variable length into a series of zero values and a subsequent non-zero value. In a preferred embodiment the variable-length decoder is constituted by the variable-length decoder which is already present for decoding the coefficients.